[.NET 6] Enable the sealer optimization in the linker

[rolfbjarne]

As the description says: enable the sealer optimization in the linker.

The difficult part lies in the fact that the only way to enable the sealer optimization is to pass the --enable-opt sealer command-line argument to the linker, and we currently have most of the logic to determine whether the sealer substep should be enabled or not in our custom linker steps.

A couple of options:

• Move much of the logic to determine if the sealer substep should be enabled out of the custom linker steps and into msbuild tasks that execute before we execute the linker. Doing this while at the same time not duplicating a lot of code is somewhat tricky, since we still need to code inside our custom linker steps.
• Ask the linker to make it possible to enable optimizations from our custom linker steps.

Ref: #9575